1. Field of the Invention
The present invention relates to a constant current circuit, and more specifically, to a constant current circuit wherein currents in equal magnitude flow in collectors (drains) of two transistors with different emitter (source) sizes and a constant current based on a voltage difference generated between bases (gates) and emitters (sources) of the transistors is output thereby.
2. Description of the Related Art
An example of a conventional constant current circuit of the above kind is shown in FIG. 1.
In the constant current circuit shown in FIG. 1, a current mirror circuit consisting of PNP transistors Q23 and Q24 is connected to the output: of a constant current generating unit consisting of resistor 21 and NPN transistors Q21 and Q22 whose emitter size ratio is 1:n1, and output current Iout is obtained by transistors Q23 and Q24 and PNP transistor Q25 which shares the common base with the transistors Q23 and Q24.
In this circuit, let Iref be a reference current flowing in the emitter of transistor Q22, I1 be a current flowing in the emitter of transistor Q21, and Iout be an output current flowing in the collector of transistor Q25. Let 1:n1 be the emitter size ratio of transistors Q21 and Q22, 1:n2 be the emitter size ratio of transistors Q24 and Q25, and R21 be a resistor connected to transistor Q22 in series. Regarding the effect of a current gain hFE of the PNP transistors, the following equations are obtained: EQU Iref=(1/R21).multidot.(K.multidot.T/q).multidot.1n(n1.multidot.I1/Iref) EQU I1=Iref.multidot.hFE/(hFE+2+n2) EQU Iout=Iref.multidot.n2.multidot.hFE/(hFE+2+n2)
where K means the Boltzmann constant, T the absolute temperature, and q a charge of electrons.
As is obvious from the above equation, output current Iout is greatly dependent on the current gain. Output current Iout is also dependent on Early voltage V.sub.A, as shown below: EQU I1=Iref.multidot.(1+V.sub.CE Q23/V.sub.A)/(1+V.sub.CE Q24/V.sub.A) EQU =Iref.multidot.(1+V.sub.CE Q23/V.sub.A)/(1+V.sub.BE Q24/V.sub.A) EQU Iout=Iref.multidot.n2(1+V.sub.CE Q25/V.sub.A)/(1+V.sub.BE 24/V.sub.A)
FIGS. 3 and 4 are diagrams showing dependency characteristics of output current Iout on hFE and on Early voltage respectively, both of which have been obtained by simulation. Dashed lines 20 in FIGS. 3 and 4 show output characteristics of this circuit.
FIG. 2 shows a configuration of another conventional constant current circuit of this kind with improved hFE dependency of the output current and comprising transconductance amplifier (TCA) circuit 6.
In the circuit configuration shown in FIG. 2, transistors Q40 and Q41 are connected sharing the common base with transistors Q31 and Q32. The emitter sizes of transistors Q31 and Q40 are the same, and so are for Q32 and Q41. Resistors R31 and R32 have the same resistance.
The collector of transistor Q40 is connected to the inverting input terminal of a differential circuit constituting TCA circuit 6. The collector of transistor Q11 is connected to the non-inverting input terminal of the differential circuit. The output of this differential circuit is connected to the collector of transistor Q32.
Operation of the conventional circuit shown in FIG. 2 will be explained below.
Collector current IC10 of transistor Q40 and collector current IC11 of transistor Q41 generate a differential current in the same magnitude as a current difference between collector current IC1 of transistor Q31 and collector current IC2 of transistor Q32. The differential current is converted into differential voltage .DELTA. Vd by resistors R33 and R34. The differential voltage is then converted into a current by the differential circuit constituting TCA circuit 6, and supplied to the collector connection point between transistors Q32 and Q34.
For example, if the current gain hFE of the PNP transistors decreases, the current gain of transistors Q33, Q34 and Q39 becomes smaller, and the base current of transistors Q33, Q34 and Q39 becomes larger. As a result, a current difference (IC2-IC1) becomes larger and so does a differential current (IC11-IC10). Therefore, TCA input voltage .DELTA.Vd becomes larger, and a TCA output current (IC6-IC7=I.sub.F 8) becomes larger and is fed back to reduce the current difference (IC2-IC1).
With such feedback control, a change in output current Iout is made to be small even in the case where the current gain hFE decreases.
As described above, among the above conventional constant current circuits, the circuit shown in FIG. 1 has the problem that the output current depends on both the current gain hFE and the Early voltage. On the other hand, the circuit shown in FIG. 2 has the problem that the output current depends on the Early voltage, although the effect of current gain fluctuation can be made small. In other words, in the case of low Early voltage, collector-emitter voltages V.sub.CE of transistors Q32, Q33, Q40, and Q41 become high when a power source voltage becomes high. This leads to increases in currents IC1, IC2, IC10, and IC11. However, since transistors Q32 and Q33 are of different conductivity type and thus have different Early voltages, currents IC1 and IC2 have increases in different magnitude.
Furthermore, the collector-emitter voltages V.sub.CE of transistors Q31 and Q40 are not the same, and accurate current difference and voltage difference are hard to obtain. As a consequence, output current Iout becomes fluctuant in response to a variance in the Early voltage or a fluctuation of the power source voltage.